Datasheet

( )
105
2 300 50 100 392
3.6 0.7
BAT Peak
I Hz nF mAp= ´ ´ ´ ´ =
´
( )
2
Boost
BAT Peak P
BAT Boost
V
I f C V
V
p
m
= ´ ´ ´ ´
´
( )
2
Capacitor Peak P
I f C Vp= ´ ´ ´
DRV8662
www.ti.com SLOS709A –JUNE 2011–REVISED NOVEMBER 2012
CURRENT CONSUMPTION CALCULATION
It is useful to understand how the voltage driven onto a piezo actuator relates to the current consumption from
the power supply. Modeling a piezo element as a pure capacitor is reasonably accurate. The equation for the
current through a capacitor for an applied sinusoid is given by Equation 3
(3)
where f is the frequency of the sinusoid in Hz, C is the capacitance of the piezo load in farads, and V
P
is the
peak voltage. At the power supply (usually a battery), the actuator current is multiplied by the boost-supply ratio
and divided by the efficiency of the boost converter as shown by Equation 4.
(4)
Substituting typical values for the variables of this equation yields a typical peak current seen by the battery with
a sine input as in Equation 5.
(5)
INPUT FILTER CONSIDERATIONS
Depending on the quality of the source signal provided to the DRV8662, an input filter may be required. Some
key factors to consider are whether the source is generated from a DAC or from PWM and the out-of-band
content generated. If proper anti-image rejection filtering is used to eliminate image components, the filter can
possibly be eliminated depending on the magnitude of the out-of-band components. If PWM is used, at least a 1
st
order RC filter is required. The PWM sample rate should be greater than 30 kHz to keep the PWM ripple from
reaching the piezo element and dissipating unnecessary power. A 2
nd
order RC filter may be desirable to further
eliminate out-of-band signal content to further drive down power dissipation and eliminate audible noise.
STARTUP/SHUTDOWN SEQUENCING
A simple startup sequence should be employed to maintain smooth haptic operation. If the sequence is not
followed, unintended haptic events or sounds my occur. Use the following steps to play back each haptic
waveform.
PWM Source
1. Send 50% duty cycle from the processor to the DRV8662 input filter. This is to allow the source and input
filter to settle before the DRV8662 is fully enabled. At the same time (or on the next available processor
cycle), transition the DRV8662 enable pin from logic low to logic high.
2. Wait 2 ms to ensure that the DRV8662 circuitry is fully enabled and settled.
3. Begin and complete playback of the haptic waveform. The haptic waveform PWM should end with a 50%
duty cycle to bring the differential output back to 0 V.
4. Transition the DRV8662 enable pin from high to low and power down the PWM source.
DAC Source
1. Set the DAC to its mid-scale code. This is to allow the source and input capacitors to settle before the
DRV8662 is fully enabled. At the same time (or on the next available processor cycle), transition the
DRV8662 enable pin from logic low to logic high.
2. Wait 2 ms to ensure that the DRV8662 circuitry is fully enabled and settled.
3. Begin and complete playback of the haptic waveform. The haptic waveform should end with a mid-scale DAC
code to bring the differential output back to 0 V.
4. Transition the DRV8662 enable pin from high to low and power down the DAC source.
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